1. It's great that you are asking this question. Marketing is an essential part of any start up -- both from creating the product that the market wants and also in promoting it.
As a marketer, it's not something I see enough of.
2. Try LinkedIn, startup networking events, social media (Twitter is great to see who is in your area), blogs, etc. You can ask for recommendations as well. Once you have money, start looking at public relations and marketing firms because they often know the industry better, dealing with multiple clients and a staff full of marketers.